Not bad for a first attempt! The flowers could do with a little more colour saturation though to make them really stand out and possibly a bit more sharpening too applied in post processing.
I like the blurred backgrounds. What lens did you use and what camera settings? Taking those kind of shots I would go into the menu and select "vivid" from the colour options and probably use an 85mm zoom setting or telephoto. A dedicated macro lens would be best but I don't have one and can't justify the expense of buying one. Nikon make a couple of good ones though at 60mm and 105mm. If you are seriously going to get into macro shots then better start saving your pesos!
I wish you well with your new camera. I am sure you will have a lot of fun with it.

I think the f number is a little high at f8. I believe the D90 has a macro programme and if you used that or full auto you would be better going into aperture priority and setting an f stop of about 4 or whatever the maximum is at the focal length you are using. I think the 18-105 lens has a max of 3.5 at 18mm and 5.6 at 105mm like most kit lenses. I don't have that particular lens so am not sure.
I don't have a D90 either but on my D80 which was the D90 predecessor you can go into the menu and adjust the settings for colour saturation and in-camera sharpening. I am sure the D90 is the same or similar. You can also use a post processing programme on your computer to alter these settings for your image file. If you don't yet have one then there are plenty on the market, Elements is fine for most people but there are alternatives. The software Nikon give away with their cameras (View NX) is only good as a raw converter and viewer. It's far too basic to be of much use although it does allow picture control changes within defined parameters.
Anyway salamat for responding to my earlier post.